This invention relates to the detection and sizing of cracks in metals using alternating current field measurement methods, and more specifically, to a sensor head and equipment to be used with such methods.
Alternating Current Field Measurement (ACFM) techniques for non-destructive testing are well-known in the industrial world. They have been extensively used in detecting cracks in metals located in inaccessible places such as underneath the sea. They have also been used to detect minute cracks in components such as aircraft skins, pipe surfaces, and the like. However, while the ACFM technique works well, there are some drawbacks with the equipment currently used in applying the techniques. To explain these drawbacks, an explanation of the ACFM technique is required.
The ACFM technique is an electromagnetic non-contacting technique which was developed to detect and size surface breaking defects occurring in different materials and through coatings of various thicknesses. The technique is based on the principle that a current induced in a material produces a magnetic field. Disturbances or singularities in the material cause variances in an otherwise relatively uniform magnetic field. By detecting and measuring these magnetic field variances, defects in the material can be found. Using ACFM, sensors are used to measure the magnetic field above the test area and thereby to detect defects. However, sensors currently being used in ACFM based equipment can only detect and size a specific type of crack. These sensors can detect and size cracks, such as stress or corrosion cracks, in test materials. However, while they can detect slanted cracks, these sensors cannot size slanted cracks nor can they determine the angle of inclination of these cracks. In this context, a straight crack is a crack that is substantially perpendicular to the surface of the material. A slanted crack is a crack that is at an angle to the surface of the material.
One such sensor is that disclosed by Topp and Lugg in EP Application 0 566 624 filed Jan. 10, 1992. In this application, the sensor is comprised of two coils at right angles whose axes intersect at a common central point. As noted above, this sensor arrangement can only detect and size straight cracks.
Accordingly, there is a requirement for a sensor arrangement and equipment which can be used with ACFM techniques to detect and size all types and configurations of cracks.
The present invention seeks to provide a sensor head in which multiple sensors are arranged to detect and size slanted cracks by detecting different components of variations in the magnetic field produced by the alternating current. A yoke is used to induce a uniform alternating current in the test material. A reference coil detects the background field strength of the magnetic field in the horizontal direction. The multiple sensor coils, the axis of each of which is in an angled relationship with the reference coil axis, detects variations in the magnetic field in the vertical plane at different angles. One of the sensor coils can be perpendicular to both the yoke axis and the reference coil axis and can detect and size straight cracks. Other sensor coils, having axes angled away from this first sensor coil, can detect and size angled cracks.
In a first embodiment, the present invention provides a sensor head for detecting variations in a magnetic field induced on the surface of a test piece by an electromagnetic elongated yoke, the sensor head comprising
a reference coil for sensing magnetic fields produced by the current, the reference coil having a longitudinal axis parallel to a longitudinal axis of the elongated yoke, and the reference coil being disposed between the yoke and the test piece;
a plurality of sensor coils for sensing variations in magnetic fields, each sensor coil having a longitudinal axis which intersects the longitudinal axis of the reference coil and each sensor coil being positioned between the reference coil and the yoke; and
mounting means for mounting the reference coil and the plurality of sensor coils in the sensor head.
In a second embodiment, the present invention provides an ACFM testing module for testing a test piece, the module comprising:
an electromagnetic elongated yoke;
a reference coil having a longitudinal axis parallel to a longitudinal axis of the yoke, the reference coil being disposed between the yoke and the test piece;
a plurality of sensor coils, each sensor coil having a longitudinal axis which intersects the longitudinal axis of the reference coil and each sensor coil being positioned between the reference coil and the yoke;
power supply means providing electrical power to the yoke;
measurement means, the measurement means being coupled separately to the reference coil and to each sensor coil; and
mounting means for mounting the reference coil and the plurality of sensor coils in a sensor head.
In a third embodiment, the present invention provides a sensor head for use with ACFM crack detection and sizing and for use with an electromagnetic elongated yoke inducing a current sheet in a test piece, the head comprising:
a reference coil having a longitudinal axis parallel to a longitudinal axis of the yoke, the reference coil being positioned between the test piece and the yoke;
at least three sensor coils, each sensor coil having a longitudinal axis which is perpendicular to the longitudinal axis of the reference coil, each sensor coil being positioned between the reference coil and the yoke; and
mounting means for mounting the reference coil and the at least three sensor coils in the sensor head;
wherein the longitudinal axes of the at least three sensor coils are located in a plane perpendicular to the longitudinal axis of the reference coil.
In a fourth embodiment, the invention provides a method of detecting and sizing defects in or near to a surface of a test piece, said method comprising:
a) energizing a yoke to induce a uniform alternating current on the surface of the test piece;
b) positioning a reference coil such that a longitudinal axis of the reference coil is parallel to a longitudinal axis of the yoke;
c) positioning at least two sensor coils with their axes at an angle to the reference coil axis;
d) moving a sensor head including the reference coil, the sensor coils and the yoke across the surface of the test piece;
e) detecting variations in electromotive force induced in the reference coil
f) measuring the variations detected in step e);
g) detecting variations in electromotive force induced in the sensor coils;
h) measuring the variations detected in step g); and
i) processing data obtained in steps f) and h) to determine the presence and the size of a defect.